Work is in progress that will allow identifidation of human pituitary cell types by light and electron microscopy, supplemented by electron microscopic immunocytochemistry, and establish their role in the production of hormones. The intermediate lobe of the rat pituitary is investigated, as a model of the ACTH/MSH cells often found in the human pars nervosa. Attempts will be made to characterize and compare the optimal milieu for adenylate cyclase activity in normal vs. goitrous rat thyroids. Preliminary results indicate that the enzyme in the former shows optimal activity at ATP 1.2 mM, Mg ions 5 mM, in the latter, at ATP 2 mM, Mg ions 12 mM.